Over a long period of time, the solar radiation has been measured with a radiation sensor which is called pyranometer, defined and categorized by ISO9060 standard. The conventional pyranometer uses a thermal sensor with a black absorbent material surface for receiving the radiation within the range of solar spectrum (main wavelength range 300 nm to 3000 nm).
In a pyranometer, it is important to obtain accurate irradiance for every incident angle of the sun light that changes depending on the time.